The present invention relates to a thermal transfer sheet capable of providing monochromatic or full-color continuous tone rendition using a thermal print head or the like and a method for fabricating the transfer sheet.
Prior art thermal transfer sheets comprise a heat resistive base composed of a condenser paper or polyethylene terephthalate (PET) film and a layer of thermally transferable ink deposited on one surface of the base. The thermally transferable ink is a mixture of a hot melt binder and pigment. A thermal head scans the transfer sheet to successively heat elemental areas of the ink layer to render them transferable to a recording sheet. For ink transfer to occur it is necessary that the heat applied to each elemental area penetrate the full thickness of the ink layer. The melted ink on each elemental area is transferred in one lump to the recording sheet. Thus, the printed ink dot represents one of two discrete tone values. Although satisfactory for printing documents, the prior art transfer sheet is disadvantageous for continuous tone rendition.
One possible solution would be to use digital density shading techniques such as the dot-pattern method. However, the digital techniques reduce image resolution and require complex circuitry.